This application claims the priority of 196 13 183.9, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method and a device for the precision turning of a workpiece made of a heat-treatable steel by way of a cutting tool.
Due to the hard cutting materials which are currently available for machining materials, extremely hard workpiece materials can sometimes be machined by turning and, in the case of unhardened materials, very high cutting speeds may be achieved. When precision turning workpieces, in many cases in contrast to grinding, coolants can be dispensed with, while nevertheless maintaining the required high surface qualities. From an energy point of view also, precision turning has advantages compared to grinding, owing to the defined tool geometry. Due to the abovementioned highly loadable cutting materials, there is now also a tendency to precision-turn workpieces in the region of hardened surface parts, i.e. so-called hard precision turning.
A drawback of precision machining of hardened workpiece surfaces, whether by grinding or by precision turning, is that as a result, owing to the machining, a certain amount of heat is introduced relatively rapidly into the workpiece. This heat has to be continuously removed again to the outside by a substantial supply of cooling liquid, in order to avoid an uncontrolled, thermally caused microstructural change in the hardened layers of material situated close to the surface. If an external cooling were dispensed with, the introduction of heat, caused by machining, into the workpiece surface would lead to a build-up of heat close to the surface and thus to an uncontrolled tempering, which reduces the desired surface hardness.
A drawback of external cooling is that considerable outlay is required in order to maintain such a cooling liquid circulation and to regenerate the circulated cooling liquid, for which reason it would be desirable to do without cooling liquid. In addition, some of the additives in the cooling liquids which are provided with various additives evaporate. In some cases, this evaporation can affect the health of the people in the work area.
An object of the present invention is to improve the basic precision turning method or the corresponding device. That is, on one hand, when precision turning hardened workpiece surfaces, there is no need to cool the machining operation by cooling liquid. The above-mentioned tempering is nevertheless avoided, or such that, on the other hand, when precision turning surfaces, which are as yet unhardened but are to be hardened, of a heat-treatable steel workpieces, there is no need for a separate heat treatment.
Starting from the basic method or the corresponding device, this object has been achieved according to the present invention by using the precision turning to cause temporary heating of a part situated close to a surface of the workpiece and locally delimited close to an immediate vicinity of a cutter of the turning tool, (b) heating the surface locally, in a region directly behind a point of action of the cutter by a laser beam, (c) moving the laser beam together with the turning tool, (d) supplying the laser beam to the point of action via a glass fiber through a passage integrated in the turning tool, (e) dimensioning the power density at the point of action of the laser beam, which is directed onto workpiece surface such that, taking into account a relative speed between the laser beam and the surface, material close to the surface is temporarily heated to the transformation temperature of the material to be machined, (f) self-quenching the material by cold volumes of the material which are situated more deeply in the workpiece with consequent hardening of the workpiece close to the surface; and (g) selecting the rotational speed of the workpiece during the precision turning so that the rotation time of the workpiece is shorter than the time for the self-quenching of workpiece parts which are heated to the transformation temperature.
Moreover, the turning tool of the present invention is provided with an integrated passage with an opening on a workpiece side, and at least one glass fiber being arranged in the passage, such that, a laser beam can be supplied through the passage, to a surface below the cutting edge of the turning tool at a distance of about 2 to 3 mm beneath the cutter, whereby with the laser beam power dimensioned such that, taking into account a relative speed between the laser beam and a workpiece surface, the material can be temporarily heated close to the surface to the transformation temperature of the material to be machined.
With the method and device of the present invention, the introduction of heat over a small area, which is caused by machining and endures only for a very short time, is utilized and built on to increase the introduction of heat by the action of a laser beam up to the transformation temperature. Then, due to a self-quenching, an ordered and defined hardening structure is formed. When precision turning workpiece parts which have already been hardened, the tempering action of the introduction of heat caused by machining is compensated for by the heat treatment according to the present invention, and when precision turning surface parts which have not yet been hardened but are to be hardened, the machined surface is simultaneously also hardened during the machining time.
Surfaces of parts to be turned, which surfaces do not need to be machined, can also be hardened by taking the cutting tool out of action there but guiding it along at a short distance from the surface to be hardened, with the laser beam switched on and the workpiece rotating.
Furthermore, it is also possible locally to harden only partially the workpiece surface over which the tool moves, whether removing metal or otherwise, by interrupting the laser beam. Specifically, it is possible in this manner not only to interrupt the hardening zone axially, but also certain peripheral regions can be left out during hardening in a targeted manner by an interruption of the beam which is rotationally synchronous and dependent on the angle of rotation.
Although DE 42 33 035 C1 describes integration of an observation window in a machining tool, which window, depending on the machine tool application, can be arranged on the tool face or on the surface below the cutting edge, this window is only intended to capture and evaluate by thermography a thermal radiation emitted by the workpiece at the machining point.
Furthermore, the simultaneous use of a laser beam at the machining point is known, for example, in turning (U.S. Pat. No. 4,229,640 or B. Wantzen: "Revolution in der Fertigungstechnik--Laserunterstutzte Warmzerspanung verhilft neuen Werkstoffen zum Durchbruch" translation: Revolution in production technology--laser-assisted thermal machining helps new materials along to breakthrough!, in the "Transfer", No. 9, 1994, pages 14-16). In this latter document, directly preceding the machining point and at a constant relative position with respect thereto, a laser beam is guided onto the outside of the as yet unmachined workpiece surface, to a certain degree into the chip root. As a result, the material is softened so that it can be machined with less resistance.
With the simultaneous use of a laser beam during machining, as shown in DD 254 348 A1, the laser beam is directed onto the outside of the already removed chip in the region of the face of the tool. The laser beam is a pulsed beam, whose point of action covers the entire cross-section of the chip and whose energy density is so high that the chip material vaporizes at the point of action during the short period of action. Consequently, the chip, which is removed from the workpiece in a continuous piece, is comminuted into short portions, to a certain extent granulated, so that it can flow and the machine can easily be kept clean. Thereby accumulations of chips cannot form, and obstructions or even blocking cannot result.
In the known laser devices described above, the laser beam was guided freely or in a rectilinear tube body and was deflected via mirrors. It is also known to guide high-energy laser beams to their point of action in flexible glass fibers as seen, for example, in U.S. Pat. Nos. 4,676,586; 4,799,755; and 4,681,396. None of the known devices or methods, either individually or when considered together, provide any indications for the person skilled in the art in the direction of precision turning hardened workpiece surfaces while obviating the need for cooling liquid.
The advantages of the method and device according to the present invention include:
Avoidance of an uncontrolled tempering during dry hard precision turning. PA1 Time-saving surface hardening of the workpieces simultaneously with the finishing or precision machining. PA1 Avoidance of hardening distortions and of resulting reworking, such as grinding or hard precision turning. PA1 Simple local hardening of certain locally limited surface parts by switching the laser beam on or off during the rotation of the workpiece. PA1 Hardness "ramps", that is to say progressive transitions from hard to unhardened material, can also be realized in a simple manner. PA1 Interference-free supply of the laser beam to the moveable point of action of the machining tool; specifically, the laser beam is not affected by parts of the machine and, conversely, the operation of the machine is not impaired by the beam-guiding elements.